A. Field of the Invention
The present invention relates to power semiconductor devices for use in power converters, in particular to IGBTs that are made using an FZ (floating zone) wafer and have bidirectional withstand capability, called bidirectional IGBTs or reverse blocking IGBTs.
B. Description of the Related Art
Conventional IGBTs (insulated gate bipolar transistors) that have a planar pn junction structure are used with a dc (direct current) power supply in their main application field of inverter circuits or chopper circuits. Since no problems occur in such application fields as long as a forward breakdown voltage is secured, obtaining reverse withstand capability has not been considered an important factor in designing and manufacturing such IGBTs.
In recent years, however, matrix converters such as a directly linked conversion circuit are being employed in semiconductor converter systems to execute an ac (alternating current) to ac conversion, an ac to dc conversion, or a dc to ac conversion. The use of bidirectional switching elements in the matrix converter for the purpose of miniaturization, reduction of weight, high efficiency, fast response, and low cost of the circuit has been studied. Therefore, IGBTs having reverse blocking capability are required in order to obtain a bidirectional switching element consisting of anti-parallel connected reverse blocking IGBTs.
FIGS. 25(a), (b), and (c) show a matrix converter circuit. FIG. 25(a) shows the circuit including the switching elements for three phases. FIG. 25(b) shows one switching element using ordinary IGBTs, while FIG. 25(c) shows a switching element using bidirectional IGBTs having bidirectional withstand capability. Conventional IGBTs 1a, 1b in the converter circuit as shown in FIG. 25(b) need diodes 2a, 2b series-connected and in the forward direction connecting to the IGBTs to secure reverse breakdown voltage because the IGBTs were not designed and produced for obtaining effective reverse blocking capability. The series-connected diodes generate large losses resulting in low conversion efficiency of the converter. Large number of device elements causes difficulties in achieving small size, light weight, and low cost of the converter. Reverse blocking IGBTs 1c and 1d as shown in FIG. 25(c) can eliminate the series-connected diodes.
FIGS. 24(a) and (b) are sectional views of an essential part of a reverse blocking IGBT. FIG. 24(a) shows the cross section when a reverse voltage is applied and FIG. 24(b) shows the cross section when a forward voltage is applied. In FIGS. 24(a) and (b), a deep p+ type isolation region 11 is formed by diffusion from front and rear surfaces of an n type FZ wafer that serves as an n− drift layer 3. Then, MOS gate structures are formed comprising a plurality of p+ base layers selectively formed in the front surface region of the n− drift , layer 3, n+ emitter region 5 selectively formed in the surface region of each of the p+ base layers 4, gate oxide films 6, gate electrodes 7, and an emitter electrode 8. After the formation of the MOS gate structure, a thickness of the n− type drift layer 3 is reduced to about 100 μm in the case of reverse withstand voltage of 600 V by removing the rear portion of the drift layer. After the thickness reduction, a p+ collector layer 9 is formed by ion implantation from the rear surface and following annealing. Thus produced IGBT device is surrounded by the heavily doped p+ isolation region 11 around the side face of the device at the dicing position 10. Consequently, a depletion layer 12 on application of a reverse voltage only extends towards the vicinity of the pn junction at the p+ collector layer 9 and the p+ isolation region 11 and does not appear at the side face of the device at the dicing position. Thus, an electric field develops only on the front surface of the device. Therefore, a sufficient reverse breakdown voltage can be attained. (See Japanese Unexamined Patent Application Publication Nos. H07-307469, 2001-185727, 2002-076017, and 2002-353454 and M. Takei et al., Proceedings of 2001 International Symposium on Power Semiconductor Devices and ICs, 2001, Osaka, Japan, pages 413-416, “600 V-IGBT with Reverse Blocking Capability”.) If a conventional IGBT that lacks this p+ isolation region 11 is reversely biased with an emitter at a ground potential and a collector at a negative potential, electric field concentration occurs at a substrate end region of a p+ collector layer, resulting in increased leakage current and insufficient reverse breakdown voltage.
Antiparallel connection as in FIG. 25(c) of the devices of FIGS. 24(a) and (b) make it possible to control forward and reverse current and to withstand application of forward and reverse voltages. Thus, the device of FIGS. 24(a) and (b) can be operated as a bidirectional device. Application of such bidirectional devices to an ac to ac converter allows direct conversion from ac to ac. Size of a converter circuit is drastically reduced as compared with a converter consisting of a converter, a capacitor, and an inverter. Consequently, the cost is substantially reduced. The bidirectional device operates as an IGBT and a free wheeling diode (FWD).
At the time of reverse recovery in the FWD operation, accumulated excess carriers are swept out by a depletion layer extending from the collector side. If the quantity of the carriers in the collector side is large, reverse recovery peak current becomes large, which is hard recovery behavior. For a reverse blocking IGBT to use as a FWD, improvement of the reverse recovery performance is essential. A method for improving the reverse recovery performance is known in which a collector layer in the rear side is formed by low temperature activation and with a low concentration (See Japanese Unexamined Patent Application Publication No. 2002-353454).
FIG. 26 is a sectional view showing a peripheral breakdown withstanding structure of an IGBT. (See Japanese Unexamined Patent Application Publication No. 2000-208768). Referring to FIG. 26, in a front surface region of n− drift layer 23 formed are p+ base layer 24 of a MOS gate structure, and p type field limit layers 25 and n type channel stopper layer 22 that are parts of a peripheral breakdown withstanding structure. Each field limit layer 25 is in contact with respective field limit electrode 27, which extends over oxide films 26 between field limit layers 25. Channel stopper layer 22 is in contact with channel stopper electrode 21, which extends in the direction toward emitter electrode 28. P+ collector layer 29 is formed in another surface region of n− drift layer 23.
A peripheral breakdown withstanding structure of usual IGBTs and FWDs is constructed so that the breakdown voltage is higher at the forward bias in which a collector electrode is at a positive potential and an emitter electrode is at a negative potential. Specific breakdown withstanding structures known in the art include a field limit layer, a field limit electrode, a combination of a field limit layer and a field limit electrode, SIPOS, and RESURF. A structure of the combination of a field limit and a field limit electrode is disclosed in Japanese Unexamined Patent Application Publication No. 2000-208768. The structure is advantageous to obtain stable long term reliability. Generally, in a high humidity environment, when negative ions enter the surface region of the oxide film of the breakdown withstanding structure, positive charges are induced on the semiconductor surface beneath the oxide film, causing a lack of uniformity in electric field distribution, thereby decreasing breakdown voltage. The structure of the Japanese Unexamined Patent Application Publication No. 2000-208768 facilitates a structure that has relatively narrow distances between the field limit layers and relatively long field limit electrodes in the region near the principal junction, which is a pn junction between the n− drift layer and the p layer in contact with the emitter electrode. The structure decreases openings between the field limit electrodes where the oxide film is exposed and inhibits invasion of the negative ions. Therefore, the adverse effect of the negative ions can be prevented.
However, distribution of equipotential lines in such a combination of field limit layer 25 and field limit electrode 27 is sensitive to the arrangement of lengths, depths, and intervals of the layers and the electrodes. For uniform distribution of electric potential and electric field strength shared by each field limit layer 25, generally the distances between field limit layers 25 must be made relatively narrow in the side of emitter electrode 28 and gradually widen towards the periphery of the element. The distances between field limit layers 25 in the region nearest to emitter electrode 28, in particular, are such that joining of built-in depletion layers at zero volt bias occurs between the adjacent p layers, which is the principal junction of the field limit layer. The distance between outermost field limit layer 25 and channel stopper layer 22 is set to be 162 μm for a 1,200 volt device that is about the diffusion length of minority carriers so that a depletion layer does not reach channel stopper layer 22. As a result, the length of the breakdown withstanding structure of the 1,200 V device is designed to be 708 μm to obtain a stable breakdown withstanding structure with little effect of surface charges.
A resistive film also has been used to achieve forward and reverse blocking characteristics. The technique uses a resistive nitride film formed on the oxide film in the breakdown withstanding structure. A minute amount of electric current flows in the resistive nitride film to attain uniform electric potential distribution and to enhance a breakdown voltage. This technique can be used in both forward and reverse directions in a reverse blocking IGBT in particular, eliminating a field limit layer and a field plate electrode. Consequently, a length of a breakdown withstanding structure comprising a resistive film can be made shorter than that of the field limit structure comprising a field limit layer and a field plate electrode. Unfortunately, a THB test (temperature humidity biased test), which is a kind of long term reliability test, demonstrated degradation of reverse blocking capability. The THB test examined long term variation of reverse blocking capability by placing a reverse blocking IGBT module in a high temperature and humidity environment of 85% RH and 125° C. and applying reverse bias voltage of 80% of the rated voltage. This degradation can be attributed to the resistive property of the nitride film that causes corrosion in the environment. The corrosion makes electric potential distribution not uniform causing an electric field concentration that degrades the blocking performance. Therefore, it is urgently required to devise a breakdown withstanding structure of a reverse blocking IGBT that performs satisfactory long term reliability.
It has been revealed that diode operation performance of the IGBT disclosed in Japanese Unexamined Patent Application Publication No. 2002-353454 cited above is not improved even if the rear collector layer is made low injection because holes are also injected from the heavily doped p+ isolation region in the diode operation. Therefore, a structure is needed that suppresses hole injection from the p+ isolation layer.
Moreover, leakage current in the reverse biased condition, in which the emitter is positive and the collector is negative as shown in FIG. 24(a), depends on emitter injection efficiency, which is a parameter to determine an open base amplification factor of the pnp transistor. The emitter injection efficiency is substantially determined by a p+ layer (not shown in FIGS. 24(a) and (b)) formed in the surface region in the p+ base layer between n+ emitter regions 5,5, the p+ layer being in contact with the emitter electrode. The p+ layer is deeper than n+ emitter region 5 and shallower than p+ base layer 4, and is doped more heavily than p+ base layer 4. Since the p+ layer is extremely heavily doped, in an amount which can be more than 1×1019 cm−3, in order to prevent latch-up, emitter injection efficiency may be higher than 0.9. As a result, the leakage current at high temperature is more than 10 mA/cm2, which is about 100 times greater than is typical. The emitter injection efficiency can be decreased by forming an n+ layer doped more heavily than n− drift layer 3 under p+ base layer 4. The n+ layer has a depth covering p+ base layer 4 in the case of a planar type, while the n+ layer is disposed between p+ base layer 4 and n− drift layer 3 in the case of a trench type. The n+ layer in the case of the planar type, however, produces a rigorous decrease in electric field intensity during off-operation, deteriorating blocking performance. Therefore, a means is needed that reduces the reverse leakage current more readily.
Since thickness of an oxide film that is a diffusion mask for forming a p+ isolation region is not great enough according to conventional technology, boron atoms eventually penetrate through the oxide film in high temperature diffusion around 1,250° C. forming a p+ layer even under the oxide film. This situation hinders formation of a normal MOS structure and an abnormal chip of IGBT may be formed that will not turn-on.
IGBTs that have reverse blocking capability must withstand high voltage in a reverse biased condition of positive emitter electrode and negative collector electrode as well as in a forward biased condition. Accordingly, a known reverse blocking IGBT comprises a structure for reverse blocking in which a p+ isolation region is formed surrounding the end portion and spanning from front surface to rear surface of the device. An IGBT having that structure, however, failed to achieve reverse breakdown voltage equivalent to forward breakdown voltage when the combination structure of field limit layers and field limit electrodes described earlier is employed.
Measurement of breakdown voltage was made on an example of a reverse blocking IGBT with a rated voltage of 1,200 V by applying forward and reverse biased voltages, and resulted in a forward breakdown voltage of 1,480 V that is satisfactory, but a reverse breakdown voltage of 1,220 V that affords an unacceptably small margin. This poor reverse blocking performance resulted because a depletion layer reaches-through to the principal junction at a reverse bias of about 1,200 V and holes enter the depletion layer generating leakage current through a path beneath the breakdown withstanding structure corresponding to the bias voltage.
As such, in the reverse biased condition, reach-through of depletion layer occurs in the region of the breakdown withstanding structure at a smaller voltage than the forward breakdown voltage. This causes a lower reverse breakdown voltage than forward breakdown voltage. There are two reasons for the reach-through of depletion layer in the reverse-biased condition. First, in the reverse-biased condition, two types of depletion layers develop: a depletion layer that develops vertically from the pn junction at the rear collector layer towards the front surface, and a depletion layer that develops laterally from the peripheral isolation region towards the principal pn junction. With increase of the applied reverse voltage, the two depletion layers pinch off and the number of electrons necessary for depletion of the drift layer decreases with increase of the voltage. This situation tends to expand the depletion layer, resulting in the above-described reach-through at a lower voltage than the forward breakdown voltage. FIG. 27 shows this situation.
Secondly, some depletion layers are joined together at zero bias condition. Depletion layers are joined together in the region from the principal pn junction to a plurality of field limit layers already at zero bias condition. When the depletion layers expand from the rear side and from the isolation layer in a reverse biased condition, the depletion layer reaches-through towards the principal junction at the time when the depletion layer from the rear face and the isolation layer arrive at the field limit layer to which the depletion layer is already expanded at zero bias.
Therefore, the reach-through of depletion layer to the principal junction around the emitter must be prevented in the reverse biased condition, and a structure is needed thereby achieving stable long term reliability.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.